The recent increase in wind power generation is likely to continue thereby raising the level of renewable energy production to unprecedented heights. A significant amount of the forthcoming wind power generation capacity is going to be off-shore, which gives rise to specific electrical connectivity considerations and component maintenance issues. As a matter of fact, operational costs of wind farms after installation are mainly driven by maintenance, with off-shore wind farms being about fifty percent more expensive than on-shore installations. This is mainly caused by increasing dependencies on weather conditions and the additional limiting factor of very special ships capable of navigating between the turbines and transporting the components that must be exchanged.
Conventionally, wind farm maintenance events are planned in distinct time steps, starting with a multi-year plan indicating that a certain percentage of the overall equipment will be exchanged per year. This results in an initial schedule that is refined and adapted over time, based on repeatedly re-evaluated equipment characteristics. On the other hand, wind farm production planning is conventionally performed on a quite distinct time-scale, in particular on a weekly and daily basis. That is, the electricity generation planning considers the previously established maintenance events only as unchallengeable constraints.
According to the patent application WO 2013044925, in order to ensure that the fatigue load limits of all wind turbine components remain within their design lifetimes, the loads experienced by a component such as bending moments, temperatures, forces or motions may be measured and the amount of component fatigue life consumed, or of complementary residual lifetime, calculated. This calculation is done for example using a well-known technique such as a rain-flow counting algorithm and Miner's rule or a chemical decay equation. Temporary over-rating may be prohibited if a given turbine component is above its target fatigue life and is sensitive to the parameter being over-rated.
The U.S. Pat. No. 8,649,911 B2 discloses assessing a plurality of sensed (electrical, mechanical, thermal, meteorological) operating parameters with respect to respective design ratings for the operating parameters (speed limits, torque limits). This includes estimating fatigue in real time based on wind history, and may result in uprating a power set point of the wind turbine when the assessment indicates that a sensed wind turbulence is lower than the estimated wind turbulence.
The U.S. Pat. No. 8,633,607 B2 discloses a method for controlling a wind farm with a plurality of wind turbines, wherein a required power is distributed among the wind turbines based on fatigue load versus power curves of single wind turbines. Determining a fatigue load versus power curve may comprise measuring wind turbine parameters (vibrations, static load). A fatigue load versus power curve may exhibit a maximum, with the fatigue load decreasing again beyond a certain power setting, which potentially favors unsymmetrical operation of wind turbines in order to reduce an average fatigue load in the wind farm.
All the above prior art approaches have in common that wind turbine operation is controlled in order to adapt to, or comply with, present or updated requirements derived from turbine component design lifetimes, and in order to avoid exceeding a design or target fatigue load for an extended period of time. Again, previously established maintenance events are considered as unchallengeable constraints.
The patent application US 20130214534 discloses a wind farm operation control system for estimating a remaining lifetime or deterioration state of a wind turbine component; estimating an income from sales of electric power under a plurality of power limit conditions equal to one of 80%, 90%, 100% of rated power; estimating maintenance cost for each of a plurality of candidate timings of performing maintenance on the component, based on the remaining lifetime of the component under each of the plurality of power limit conditions; and selecting a power limit condition that maximizes income obtained from the wind farm. The power limit conditions introduced do represent plant-wide uniform and time-invariant decision variables.